On 9/23/22 10:34, Marek Polacek wrote:
On Thu, Sep 22, 2022 at 06:14:44PM -0400, Jason Merrill wrote:
On 9/22/22 09:39, Marek Polacek wrote:
To improve compile times, the C++ library could use compiler built-ins
rather than implementing std::is_convertible (and _nothrow) as class
templates.  This patch adds the built-ins.  We already have
__is_constructible and __is_assignable, and the nothrow forms of those.

Microsoft (and clang, for compatibility) also provide an alias called
__is_convertible_to.  I did not add it, but it would be trivial to do
so.

I noticed that our __is_assignable doesn't implement the "Access checks
are performed as if from a context unrelated to either type" requirement,
therefore std::is_assignable / __is_assignable give two different results
here:

    class S {
      operator int();
      friend void g(); // #1
    };

    void
    g ()
    {
      // #1 doesn't matter
      static_assert(std::is_assignable<int&, S>::value, "");
      static_assert(__is_assignable(int&, S), "");
    }

This is not a problem if __is_assignable is not meant to be used by
the users.

That's fine, it's not.
Okay then. libstdc++ needs to make sure then that it's handled right.

This patch doesn't make libstdc++ use the new built-ins, but I had to
rename a class otherwise its name would clash with the new built-in.

Sigh, that's going to be a hassle when comparing compiler versions on
preprocessed code.

Yeah, I guess :/.  Kind of like __integer_pack / __make_integer_seq.

--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -3697,6 +3697,12 @@ diagnose_trait_expr (tree expr, tree args)
       case CPTK_HAS_UNIQUE_OBJ_REPRESENTATIONS:
         inform (loc, "  %qT does not have unique object representations", t1);
         break;
+    case CPTK_IS_CONVERTIBLE:
+      inform (loc, "  %qT is not convertible from %qE", t2, t1);
+      break;
+    case CPTK_IS_NOTHROW_CONVERTIBLE:
+       inform (loc, "  %qT is not %<nothrow%> convertible from %qE", t2, t1);

It's odd that the existing diagnostics quote "nothrow", which is not a
keyword.  I wonder why these library traits didn't use "noexcept"?

Eh, yeah, only "throw" is.  The quotes were deliberately added in
<https://gcc.gnu.org/pipermail/gcc-patches/2019-May/522333.html>.  Should
I prepare a separate patch to use "%<noexcept%>" rather than "%<nothrow%>"?
OTOH, the traits have "nothrow" in their names, so maybe just go back to
"nothrow"?

The latter, I think. Or possibly "no-throw". I guess -Wformat-diag wants "nothrow" quoted because of the attribute of that name.

--- a/gcc/cp/method.cc
+++ b/gcc/cp/method.cc
@@ -2236,6 +2236,37 @@ ref_xes_from_temporary (tree to, tree from, bool 
direct_init_p)
     return ref_conv_binds_directly (to, val, direct_init_p).is_false ();
   }
+/* Return true if FROM can be converted to TO using implicit conversions,
+   or both FROM and TO are possibly cv-qualified void.  NB: This doesn't
+   implement the "Access checks are performed as if from a context unrelated
+   to either type" restriction.  */
+
+bool
+is_convertible (tree from, tree to)

You didn't want to add conversion to is*_xible?

No, it didn't look like a good fit.  It does things we don't need, and
also has if VOID_TYPE_P -> return error_mark_node; which would be wrong
for __is_convertible.

I realized I'm not testing passing an incomplete type to the built-in,
but since that is UB, I reckon we don't need to test it (we issue
"error: invalid use of incomplete type").

But your patch does test that, in the existing call to check_trait_type from finish_trait_expr?

The patch is OK.

Jason

Reply via email to